Isn't Spirit-based regen figured out through a formula that also looks at your mana pool?

yes, but not linear by a longshot.

Currently 1 point of spirit yields 5 times more mana return than 1 point of int.

As gearlevel rises, the disparity goes up.

spirit regen rises with the square root of int, linearly with spirit. So at higher gear levels, another point of int will give you less mp5. And at higher gear levels, another point of spirit will give you more mp5 than it does now.

Currently 1 point of spirit yields 5 times more mana return than 1 point of int.

As gearlevel rises, the disparity goes up.

spirit regen rises with the square root of int, linearly with spirit. So at higher gear levels, another point of int will give you less mp5. And at higher gear levels, another point of spirit will give you more mp5 than it does now.

Yea but with all that said mana and mana regen shouldn't really be an issue for any firelands geared disc priest ( I know the topic wasn't about disc priests but we all know holy need a lot of spirit anyways ), I'm progressing heroics at the moment with 1200 spirit, 6600 int + 2300 int from procs and trinket sure I get low sometimes but if things go as planned mana is in no way an issue and as far as I can tell not many disc priests are running with just 1k spirit from gear and enchants.

Which brings us back to the original post which I think is about what on proc enchant is actually better in the long run, I really do think it comes down to;

Do you need regen, y / n - if yes take spirit procs - if no take int. I don't think it needs to be complicated and all math orientated to figure out which one you need more fact of the matter is you will always gain more regen from spirit procs due to not having to time the proc with regen abilities.

I've run math on my Holy Priest a couple of times, and I keep reaching the conclusion that Int procs (Both Power Torrent and Lightweave Embroidery) sim ahead of the spirit counterparts, so instead of a rule of thumb, I would say that it depends on your current gear and very much your mana pool of course.

As a disc priest, Rapture regens a percentage of your total mana. Therefore, the higher your mana pool, the more mana you get back from rapture procs. A good discipline priest will line up his int procs with rapture for the most mana returned in an intense fight.

Spirit does one thing and one thing only while intellect does several.

I think some people have misunderstood the point of my OP. I think intellect is superior. I come from the standpoint of a very experienced spell caster and I know the power of Intellect very well. What I say is that - for the special case of healing, where most of the time it does not require "max HPS" or even HPS at all - Intellect is superior when it is passive. When it procs randomly spirit will give its benefit 100% of the time (with a slight exception if it's done at the very beginning of the fight or at a special phase). Intellect may proc - and it will proc - at times that it is not that much needed.

Sure, you can be lucky and sure you can be lining up spells and sure you may know a certain encounter perfectly and be good at line ups but for the average situation, if intellect is only slightly "mathematically" better - I'm not talking about huge differences, there the math will not be easily questioned - then the spirit will get a huge advantage due to its high reliability.

Also, I understand that certain types of healing will benefit more easily from random procs. E.g. a tank healer that never stops "spam" healing a constant rotation will approach a "Patchwerk DPS" state, where intellect WILL be used when randomly procced, but such cases are rare and even then intellect may not line up correctly. e.g. getting a Shield up without the proc and getting the proc on a Heal, which won't be that great.